Apparatus for rolling strip metal



May 31, 1966 J. FRANEK 3,253,445

APPARATUS FOR ROLLING STRIP METAL Filed June 24, 1963 5 Sheets-Sheet 1 Inventor FHANEK Home y 5 May 31, 1966 J. FRANEK APPARATUS FOR ROLLING STRIP METAL 5 Sheets-Sheet 2 Filed June 24, 1963 Inventor J'OZEF' FRANEK Attorneys J. FRANEK APPARATUS FOR ROLLING STRIP METAL May 31, 1966 5 Sheets-Sheet 5 Filed June 24, 1963 Inventor J'AZEF FRANEK /Znaa M Attorney;

y 1966 .1. FRANEK APPARATUS FOR ROLLING STRIP METAL 5 Sheets-Sheet 4- Filed June 24, 1963 Inventor J'OZEF FRAME K M Attorneys May 31, 1966 J. FRANEK APPARATUS FOR ROLLING STRIP METAL 5 Sheets-Sheet 5 Filed June 24, 1963 Inventor J'OZEF FRANEK Attorneys United States Patent 3,253,445 APPARATUS FOR ROLLING STRIP METAL Jozef Franck, London, England, assignor to The Metal Box Company Limited, London, England, a British company Filed June 24, 1963, Ser. No. 289,953 Claims priority, application Great Britain, July 9, 1962, 26,317/62 Claims. (Cl. 72-464) This invention relates to the treatment of strip metal of ferrous or non-ferrous base, as well as ferrous base metal having a protective coating for example of tinplate, which is rolled to a predetermined thickness which may be of the order of 0.010 inch or more or may be a foil.

As is understood in the art, metal which is rolled to a thickness of up to about 0.01 inch to form a long strip may suffer to a lesser or greater extent from a number of defects in the geometry thereof such defects being known in the art as:

(a) Camber or edge wander, which is the greatest deviation of the longitudinal edges of the strip from a straight line and which does not permit the cutting from the strip of sheets with right angles,

(b) Wavy edges, which are due to compressive stresses near the edges of the strip and tensile stresses in the centre of the strip, and which cause sheets cut from the strip not to be flat so that difficulty occurs in feeding the sheets during printing, lacquerin-g, and stamping thereof,

(c) Buckle or blisters in the centre of the strip, which are usually caused by the presence of compressive stresses in the middle of the strip and tensile stresses near the longitudinal edges of the strip,

(d) Seaweed effect, which is a combination of defects and (e) Buckled edge, which is usually due to damage by engagement of the strip with guide elements, and

(f) Dents or creases, which are usually caused by damage to the strip after coiling thereof.

Besides the above defects, change in some physical properties may occur due to aging. This can be detected by subjecting a specimen of the strip material to the tensile test. The aged strip can be identified by the presence of the yield point discontinuity on the stress-strain curve. Strip suffering from such a defect in the physical properties cannot be deformed plastically in a uniform manner with complete success. For example when aged tinplate is employed for the manufacture of an open top can body, a condition known in the art as fluting or panelling will often occur and the cylinder may be polygonal instead of smoothly circular. Can ends made from aged tinplate may have stretcher strain marks (known as Leuder bands) which indicate a return of yield point discontinuity and a decrease in the formability. Such can ends will be geometrically imperfect and dimensionally unstable, especially when deep drawing, or irregular or non-circular shapes are involved.

Normally in strip metal manufacture skin passing and temper rolling are employed as corrective operations to reduce physical defects and to mask the yield point discontinuity. This is done largely by high compression stresses imparted by the rolls and moderate strip tension, resulting in a characteristic stress pattern Within the thickness of the strip which is in compression near the surfaces and tension in the middle. The strip usually undergoes further corrective treatment by a roller leveller before being cut into sheets.

The stress pattern in temper rolled strip is upset by aging and while the known methods of roller levelling applied to the strip immediately before cutting thereof into sheets may correct the stress pattern if the roll diameter is small enough it is found, particularly in the case of 3,253,445 Patented May 31, 1966 tinplate used in the manufacture of open top cans, and which has a thickness less than 0.010 inch, that the effect of the roller levelling is negligible in correcting the defects in the geometry of the strip and in the physical properties thereof.

It is one object of the present invention to provide apparatus for effecting a predetermined plastic tensile strain combined with compression which perm-its correction of the geometrical defects of a metal strip and imparts the desired physical properties'to the strip in a controlled manner. According to the amount of the predetermined tensile strain and compression applied to the strip the strip will be subjected either to temper stretch levelling or to temper rolling.

According to the invention there is provided apparatus for stretching strip metal, comprising rotatable entrance and exit bridle rollers supported for rotation at positions spaced apart along the path of a strip to be stretched and around which the strip is passed, rotatable pinch rollers operable to press the strip against the bridle rollers, first and second rotatable tensioning rollers around which the strip passes during movement thereof from the entrance bridle roller to the exit bridle roller and between which the strip is gripped, said second tensioning roller being rotatable with a circumferential velocity greater than that of the first tensioning roller thereby locally to increase the linear speed of the strip toimpart a permanent tensile strain thereto, adjustable pressure-applying means co-operating with the first tensioning roller to apply to the strip a predetermined local compressive stress lower than the yield stress of the strip while the strip is subjected to local longitudinal tensioning by the first and second tensioning rollers, and a train of gears operable to effect simultaneous rotation of all of said rollers.

In order that the invention may be clearly understood some embodiments thereof Will now be described, by way of example, with reference to the accompanying drawings, in which:

FIGURE 1 illustrates diagrammatically one form of apparatus for carrying the invention into effect,

FIGURE 2 illustrates diagrammatically a second form of apparatus for carrying the invention into effect,

FIGURE 3 illustrates a train of gears for driving rollers employed in the apparatus of FIGURE 2,

FIGURES 4A and 4B together are an elevation, partly in section, of a .part of a machine embodying the apparatus shown diagrammatically in FIGURES 2 and 3,

FIGURE 5 is a section on line V-V, FIGURE 4B, and

FIGURE 6 is a section on line VI'-VI, FIGURE 43.

In the drawings like reference numerals refer to like or similar parts.

Referring to FIGURE 1, temper stretch levelling of a strip 1 of metal is effected by moving the strip lengthwise and at a predetermined position along the path of the strip longitudinally tensioning the strip gradually round bridle rollers and locally increasing the linear speed there of to impart a permanent tensile strain to the strip While compressing the strip from opposite sides thereof to avoid the formation of undesired stretcher strain marks on-the strip and to transfer the required finish of temper rolls on to the surface of the strip.

The strip I at the said predetermined position enters between an entrance bridle roller 2, rotatable with a shaft 3, and a first pair of pinch rollers 4, 5 which are carried by a carrier 6 and press the strip against the bridle roller 2. The pinching force exerted by the pinch rollers 4,

5 may be applied by hydraulic or mechanical means and as illustrated in FIGURE 1 is applied by means of a compression spring '7 and adjusting screw 8.

On leaving the bridle roller 2 the strip 1 is Wrapped round a first tensioning roller 9 and then in turn round a second tensioning roller 10 and a second, or exit, bridle roller 11, rotatable with a shaft 12, against which it is pressed by a second pair of pinch rollers 13, 14 carried by a carrier 15 provided with a compression spring 16 and adjusting screw 17. The roller 9 is rotatable with a shaft 18 and roller 10 is rotatable with a shaft 19 carried by a carrier 20 provided with suitable means arranged to permit roller 10 to be moved towards and away from roller 9. While passing between rollers 9 and 10 the strip 3. is subjected to a local compressive stress lower than the yield stress but high enough to prevent the formation of stretcher strain marks (Lueder bands) and the pressure is exerted through the carrier 20 by hydraulic or mechanical means. As shown in FIGURE 1 the pressure is exerted by a compression spring 21 and an adjusting screw 22. The strip 1 while being locally compressed between rollers 9 and 10 is subjected to a local stretching or tensioning strain by means of a difference between the circumferential velocities of the rollers 9, 10, the circumferential velocity of roller 10 being greater than that of roller 9.

In the embodiment of the invention being described the rollers 2, 9, 10 and 11 are each of 20 inches nominal diameter and the pinch rollers 4, 5 and 13, 14 each have a diameter of 12 inches. The rollers are positively rotated by a train of gears, not shown, the pinch rollers 4, 5, 13 and 14 having a circumferential speed equal to that of the bridle rollers 2 and 11. The rollers 2, 9 are geared in 1:1 ratio and similarly the rollers 10, 11 may be also geared in 1:1 ratio to maintain equal angular velocities.

The diameters of the tensioning rollers 9, 10 and the gearing between them is selected in a manner such as to result in the desired difference between the circumferential velocities of the rollers, this difference, being between 0.5% and 5%. If the strip is to be subjected only to a slight degree of stretch the ratio of the gears driving rollers 9 and 10 may be 1:1 and the difference in circumferential velocity of the rollers obtained by making the diameter of roller 10 greater than that of roller 9. On the other hand, if a greater degree of stretch is required this will be obtained by selecting a suitable ratio for the gears rotating the rollers 9, 10 and by making the diameter of roller 10 greater than that of roller 9. When the apparatus is used for effecting a temper stretch levelling process the strip is subjected to a tensile strain below 2% combined with the above-mentioned local compressive stress less than the yield stress, but when the apparatus is used for effecting a temper stretch rolling process the strip is subjected to a tensile strain of 2% or more up to about 5% combined with the above-mentioned compressive stress less than the yield stress. The strip 1 while being locally compressed between the rollers 9, 10 undergoes a predetermined plastic tensile strain due to the greater circumferential velocity of the roller 10 as compared with that of roller 9.

To ensure uniform processing of the strip 1 the tensioning rollers 9, 10 and the pinch rollers are crowned to compensate for deflection but as the process is mainly a stretching process the rolling forces are small and crowning will be small and dependent on the width and material of the strip and the dimensions of the rolls. For example, the crowning of the pinch rolls 4, 5 and 13, 14 may be of the order of 0.030 inch and that of the tensioning rolls may be of the order of 0.006 inch.

The diameter of the first tensioning roller 9 is slightly greater, about 0.05% greater, than that of the entrance bridle roller 2 and the diameter of the second tensioning roller 10 is slightly greater than that of the exit bridle roller 11 to compensate for the gradually increasing elastic tensile strain in the strip 1 caused by the action of the bridles.

Referring to FIGURES 2 to 6, the apparatus therein illustrated operates in a manner similar to that of the apparatus described with reference to FIGURE 1 but is designed to meet the conditions of low coefficient of friction which occurs in the case of lubricated strip material fed at high linear speed to highly finished rolls. Such conditions are likely to occur when levelling tinplate strip fed at a velocity of about 900 ft./ min.

In this embodiment of the invention the strip 1 approaches the single pinch roller 5 substantially horizontally and is wrapped round it for about 90% to pass between the pinch roller 5 and entrance bridle roller 2 and between the bridle roller 2 and an auxiliary entrance bridle roller 2a. After leaving roller 2a the strip is passed round and between the tensioning rollers 9, 10 and is subjected to the above-mentioned local compressive stress and local stretching or tensioning strain and then passes round an auxiliary exit bridle roller 11a, the exit bridle roller 11, and a single pinch roller 14.

The rollers 5, 2a, 11a and 14 are respectively rotatable with shafts 23, 24, 25 and 26 and FIGURE 3 illustrates the train of gears for rotating the various rollers. From FIGURE 3 it will be seen that the tensioning rollers 9, 10 are rotated by meshing gears 27, 28; rollers 2 and 2a by meshing gears 29, 30; rollers 11, 11:: by meshing gears 31, 32; rollers 5 and 14 by gears 33, 34 meshing respectively with gears 30 and 31; and idler gears 35, 36 mesh respectively with the gears 27, 30 and 27, 32.

The rollers 2, 2a, 9, 10, 11a and 11 have a nominal diameter of 20 inches but the rollers 2, 2a and 9 are of lesser diameter than that of the nominal diameter to permit the predetermined stretch of the strip as described above. The pinch rollers 5 and 14 have a nominal diameter of 16 inches. The diameters of the individual rollers are corrected according to the elastic strain in the strip so as to minimise slip between the strip and the highly finished surfaces of the rollers.

The number of teeth of the gears 33 and 34 is proportional to the diameters of the pinch rollers 5 and 14 and the gears 30, 29 and 27 respectively have the same number of teeth proportional to the diameters of the rollers 2, 2a and 9. The gears 28, 31, 32 may have the same number of teeth as gears 30, 29 and 27 when the increased circumferential velocity of roller 10 is achieved by the diameter of the roller 10 being greater than that of roller 9. When, however, a greater degree of stretch is required the number of teeth on the gears 28, 31, 32 will be less than that of the gears 30, 29, 27. The idler gears 35, 36 transfer the drive between the bridle rollers and the tensioning rollers.

FIGURES 4 to 6 illustrate a part of a machine embodying the apparatus illustrated diagrammatically in FIGURES 2 and 3.

From FIGURE 5 is will be seen that the shafts 3, 12, 18, 19, 24 and 25, are supported in identical bearing chocks 37 mounted between top and bottom machine frame elements 38. A ichock 37, shown in FIGS. 4B and 5 but not visible in FIG. 6, is provided for each end of the shaft for each roller. The chocks 37 for the rollers 2, 10, 4 and 11 are positively attached to the frame elements by gibs 39 and adjustable distance blocks 40. The chocks 37 for the rollers 2a, 9 and 11a are connected to pairs of parallel links 41 supported by blocks 42 located in grooves 43 in side frame elements 44, and the rollers 2a, 9 and 11a carried by these chocks are urged towards their opposing rollers 2, 10, and 11 by hydraulic devices 45, known as load cells, acting through distance blocks 46. The parallel links 41 act to take up the side thrust or horizontal component of the strip tension reaction of the strip being processed. To facilitate the easy withdrawal of the roller assemblies the bearing chocks are provided with wheels 47 which run on rails 48 carried by the frame elements 44.

From FIGURES 2 and 6 it will be seen that the tensioning rollers 9, 10 are superimposed one on the other and are located between the pairs 2, 2a; 11, 11a of bridle rollers.

To prevent damage of the hardened surfaces of the rollers each chock 37 is provided with adjustable distance wedges 49. The side frame elements 44 locate the roller assemblies sideways and the elements 44 are located between the top and bottom frame elements 38 which are clamped together by bolts 50 which resist the rolling forces exerted by the load cells 45.

The pinch rollers 5, 14 are carried by bearing chocks 51 pivoted at 52 to carriages 53 having wheels 54 which run on a rail track 55 carried, see FIGURE 5, one by the top frame 38 and one by an end side frame 44. This method of mounting the chocks 51 facilitates the easy withdrawal of the pinch roller units from the frames. The pinch force is applied by links 56 pivoted at 57 in yokes 58 and carrying a roller 59 which engages a contoured portion 60 of the chock 51. The links 56 are operated by the piston rod 61 of a hydraulic cylinder 62. At one end thereof the yoke 58 is mounted on a pivot 63 and is loaded by loa-d cells 64. The other end of the yoke 58 is attached to an adjustable tension bolt 65 which shares the pinch roll reaction force with non-adjustable bolts 66, FIGS. 4B and 6. Adjustable stops 6'7 are provided to prevent damage to the hardened surfaces of the pinch rollers 5, 14 and bridle rollers 2a, 11a.

The gears 29, 30; 27, 28; and 31, 32 are all driven through connecting gear coupling arrangements, two of which are indicated at 68, FIGURE 4A, which provide the drive to the rollers which may vary in diameter and thus in the, distance between the centres thereof. 7

To facilitate threading of the strip 1 through the apparaus guides 69, FIGURE 6, are provided.

When the apparatus is employed for temper stretch levelling the strip is subjected by the tensioning rollers 9, to a local predetermined tensile strain less than 2% combined with a local compressive stress lower than the yield stress but high enough to prevent the formation of stretcher strain marks (Lueder hands).

When the apparatus is employed for temper stretch rolling the strip is subjected by the tensioning rollers 9, 10 to a local predetermined tensile stress of between 2% and 5% combined With a local compressive stress lower than the yield stress but high enough to prevent the formation of stretcher strain marks (Lueder bands).

It is found when effecting temper stretch leveling or temper stretch rolling by the apparatus according to the invention that not only is the formation of stretch-er strain marks (Lueder bands) prevented but that a preferred pattern of stresses within the thickness of the strip is returned or imparted, the yield point discontinuity is masked, and the temper of the strip is imparted or corrected and in some instances is slightly increased. The ductility of the strip is improved as is evident by the absence of stretcher strain marks in stampings made from the treated strip. Furthermore, the surface of the strip is treated by the degree of finish of the tensioning rollers 9, 10 and in many instances this results in an improvement in the surface appearance of the treated strip even when, prior to treating, the strip is severely dented or creased.

It will be understood that in each of the forms of apparatus described herein due to the extreme tensions in the strip equal to the yield stress and multiplied by the area of the cross-section of the strip, the exit brid le has to provide a large amount of work because the strip tension acts opposite to the direction of strip movement. A high percentage of the work is, however, returned to the apparatus by the entrance bridle because both strip tension and movement are inthe same direction. The work is transferred by the gears, and the driving motor, not shown, supplies the necessary work for stretching.

I claim:

1. Apparatus for temper stretch levelling or temper stretch rolling strip metal, comprising rotatable entrance and exit bridle rollers supported for rotation at positions spaced apart along the path of a strip and around which the strip is passed, rotatable pinch rollers operable to press the strip against the bridle entrance bridle roller to the exit bridle roller and between which the strip is gripped, said second tensioning roller being rotatable with a circumferential velocity greater than that of the first tensioning roller thereby locally to increase the linear speed of the strip to impart a permanent tensile strain thereto, adjustable pressure-applying means cooperating with the first tensioning roller to apply to the strip a predetermined local compressive stress lower than the yield stress of the strip while the strip is subjected to local tensioning by the first and second tensioning rollers, and a train of gears operable to effect simultaneous rotation of all of said rollers.

2. Apparatus for temper stretch levelling or temper stretch rolling strip metal, comprising rotatable entrance and exit bridle rollers'supported for rotation at positions spaced apart along the path of a strip and around which the strip is passed, rotatable pinch rollers operable to press the strip against the bridle rollers, first and second c0ntra-rotatable tensioning rollers around which the strip passes during movement thereof from the entrance bridle roller to the exit bridle roller and between which the strip is gripped, said second tensioning roller being rotatable with a circumferential velocity greater than that of the first tensioning roller thereby locally to increase the linear speed of the strip to impart a permanent tensile strain thereto, one of said tensioning rollers being rotatable in fixed bearings and the other in movable bearings permitting movement of said other tensioning roller towards and away from said one tensioning roller, adjustable pressure-applying means operable to urge said other tensioning roller towards said one tensioning roller to apply to the strip a local predetermined compressive stress lower than the yield stress of the strip while the strip is subjected to local tensioning by the first and second tensioning rollers, and a train of gears operable to effect simultaneous rotation of all of said rollers.

3. Apparatus according to claim 2 in which said movable bearings are carried by parallel links supported by stationary frame elements to accommodate the horizontal component of the strip tension reaction.

4. Apparatus according to claim 2 in which each pinch roller is rotatable in pinch roller bearings supported for pivotal movement towards and away from the bridle roller associated therewith and including for each pinch roller a stationary yoke, links pivotally connected to said yoke, a roller carried by said links and engaging contoured portions of the pinch roller bearings, and a hydraulic cylinder having a piston rod connected with said links to effect rocking of the pinch roller bearings about the pivots therefor by co-operation between the roller carried by the links and said contoured portions of the pinch roller bearings.

5. Apparatus for temper stretch levelling or temper stretch rolling strip metal, comprising rotatable entrance and exit bridle rollers supported for rotation at positions spaced apart along the path of a strip and around which the strip is passed, rotatable pinch rollers operable to press the strip against the bridle rollers, said pinch rollers being peripherally crowned, first and second rotatable tensioning rollers around which the strip passes during movement thereof from the entrance bridle roller to the exit bridle roller and between which the strip is gripped, said second tensioning roller being rotatable with a .circumferential velocity greater than that of the first tensioning roller thereby locally to increase the linear speed of the strip to impart a permanent tensile strain thereto, adjustable pressure-applying means co-operating with the first tensioning roller to apply to the strip a predetermined local compressive stress lower than the yield stress of the strip while the strip is subjected to local tensioning by the first and second tensioning rollers, and a train of gears operable to effect simultaneous rotation of all of said rollers.

6. Apparatus for temper stretch levelling or temper stretch rolling strip metal, comprising rotatable entrance and exit bridle rollers. supported for rotation at positions spaced apart along the path of a strip and around which the strip is passed, rotatable pinch rollers operable to press the strip against the bridle rollers, first and second rotatable tensioning rollers around which the strip passes during movement thereof from the entrance bridle roller to the exit bridle roller and between which the strip is gripped, said first tensioning roller being peripherally crowned to compensate for deflection of the tensioning rollers and said second tensioning roller being rotatable with a circumferential velocity greater than that of the first tensioning roller thereby locally to increase the linear speed of the strip to impart a permanent tensile strain thereto, adjustable pressure-applying means cooperating with the first tensioning roller to apply to the strip a predetermined local compressive stress lower than the yield stress of the strip while the strip is subjected to local tensioning by the first and second tensioning rollers, and a train of gears operable to effect simultaneous rotation of all of said rollers.

7. Apparatus for temper stretch levelling or temper stretch rolling strip metal, comprising an entrancebridle roller and an auxiliary entrance bridle roller supported for rotation in superimposed co-operating relation, an exit bridle roller and an auxiliary exit bridle roller supported for rotation in superimposed co-operating relation, said entrance and exit bridle rollers being located at positions spaced apart along the path of a strip and the strip being passed around and between the paired rollers,

fixed bearings in which the entrance and exit bridle rollers are rotatable, movable bearings in which the auxiliary entrance and exit bridle rollers are rotatable, said movable bearings permitting movement of the auxiliary rollers towards and away from the entrance and exit bridle rollers with which they respectively co-operate, pressureapplying means co-operating with said movable bearings to urge the auxiliary bridle rollers towards the entrance and exit bridle rollers to effect pinching of the strip between the paired rollers, first and second contrarotating tensioning rollers around which the strip passes during movement thereof from the entrance bridle roller to the exit bridle roller and between which the strip is gripped, said second tensioning roller being rotatable with a circumferential velocity greater than that of the first tensioning roller thereby locally to increase the linear speed of the strip to impart a permanent tensile strain thereto, one of said tensioning rollers being rotatable in fixed tensioning-roller bearings and the other in movable tensioning-roller bearings permitting movement of said other tensioning roller towards and away from said one adjustable tensioning roller, tensioningroller pressure-applying means operable to urge said other tensioning roller towards said one tensioning roller to apply to the strip a local compressive stress lower than the yield stress of the strip while the strip is subjected to local tensioning by the first and second tensioning rollers, and a train of gears operable to effect simultaneous rotation of all of said rollers.

8. Apparatus according to claim 7 in which said movable tensioning-roller bearings are carried by parallel links supported by stationary frame elements to accommodate the horizontal component of the strip tension reaction.

9. Apparatus according to claim 7 in which each pinch roller is rotatable in pinch roller bearings supported for pivotal movement towards and away from the bridle roller associated therewith and including for each pinch roller a stationary yoke, links pivotally connected to said yoke, a roller carried by said links and engaging contoured portions of the pinch roller bearings, and a hydraulic cylinder having a piston rod connected with said links to effect rocking of the pinch roller bearings about the pivots therefor by co-operation between the roller carried by the links and said contoured portions of the pinch roller bearings.

10. Apparatus according to claim 8 in which said pinch roller is rotatable in pinch roller bearings supported for pivotal movement towards and away from the bridle roller associated therewith and including for each pinch roller a stationary yoke, links pivotally connected to said yoke, a roller carried by said links and engaging contoured portions of the pinch roller bearings, and a hydraulic cylinder having a piston rod connected with said links to effect rocking of the pinch roller bearings about the pivots therefor by co-operation between the roller carried by the links and said contoured portions of the pinch roller bearings.

References Cited by the Examiner UNITED STATES PATENTS 2,163,504 6/1939 Thomas 72-205 2,166,418 7/1939 McBain -60 2,178,674 11/1939 Simons 72-205 2,332,796 10/1943 Hume 72-205 2,392,323 1/1946 Koss 72-205 2,526,296 10/1950 Stone 80-35 2,567,770 9/1951 Heller 72-164 2,735,474 2/1956 Sherwood 72-378 2,742,949 4/1956 Nilsson 72-161 2,857,655 10/1958 Greenberger 72-205 3,153,696 10/1964 Blanchard 72-378 CHARLES W. LANHAM, Primary Examiner. 

1. APPARATUS FOR TEMPER STRETCH LEVELLING OR TEMPER STRETCH ROLLING STRIP METAL, COMPRISING ROTATABLE ENTRANCE AND EXIT BRIDLE ROLLERS SUPPORTED FOR ROTATION AT POSITIONS SPACED APART ALONG THE PATH OF A STRIP AND AROUND WHICH THE STRIP IS PASSED, ROTATABLE PINCH ROLLERS OPERABLE TO PRESS THE STRIP AGAINST THE BRIDLE ENTRANCE BRIDLE ROLLER TO THE EXIT BRIDLE ROLLER AND BETWEEN WHICH THE STRIP IS GRIPPED, SAID SECOND TENSIONING ROLLER BEING ROTATABLE WITH A CIRCUMFERENTIAL VELOCITY GREATER THAN THAT OF THE FIRST TENSIONING ROLLER THEREBY LOCALLY TO INCREASE THE LINEAR SPEED OF THE STRIP TO IMPART A PERMANENT TENSILE STRAIN THERETO, ADJUSTABLE PRESSURE-APPLYING MEANS COOPERATING WITH THE FIRST TENSIONING ROLLER TO APPLY TO THE STRIP A PREDETERMINED LOCAL COMPRESSIVE STRESS LOWER THAN THE YIELD STRESS OF THE STRIP WHILE THE STRIP IS SUBJECTED TO LOCAL TENSIONING BY THE FIRST AND SECOND TENSIONING ROLLERS, AND A TRAIN OF GEARS OPERABLE TO EFFECT SIMULTANEOUS ROTATION OF ALL OF SAID ROLLER. 